Hydrogen possesses the NFPA 704's highest rating of 4 on the flammability scale because it is flammable when mixed even in small amounts with ordinary air; ignition can occur at a volumetric ratio of hydrogen to air as low as 4% due to the oxygen in the air and the simplicity and chemical properties of the reaction. However, hydrogen has no rating for innate hazard for reactivity or toxicity. The storage and use of hydrogen poses unique challenges due to its ease of leaking as a gaseous fuel, low-energyignition, wide range of combustible fuel-air mixtures, buoyancy, and its ability to embrittle metals that must be accounted for to ensure safe operation. Liquid hydrogen poses additional challenges due to its increased density and the extremely low temperatures needed to keep it in liquid form.

For over 40 years, industry has used hydrogen in vast quantities as an industrial chemical and fuel for space exploration. During that time, industry has developed an infrastructure to produce, store, transport and utilize hydrogen safely.

Hydrogen gas is an extremely high-powered fuel. It burns with incredible speed and can produce incredible force and is used mostly for applications that require vast amounts of instantaneous power - such as rocketry and spaceflight. For instance, it was used to power the Space Shuttle.

Liquid hydrogen is sometimes used as an extremely condensed hydrogen fuel.

Gaseous hydrogen can be used as a coolant for electric generators in power stations. This is because of its high thermal conductivity and low "windage", so reducing frictional and turbulence losses.

Hydrogen is also used as a feedstock in industrial processes including production of ammonia and methanol.

The current ANSI/AIAA standard for hydrogen safety guidelines is AIAA G-095-2004, Guide to Safety of Hydrogen and Hydrogen Systems.[4] As NASA has been one of the world's largest users of hydrogen, this evolved from NASA's earlier guidelines, NSS 1740.16 (8719.16).[5] These documents cover both the risks posed by hydrogen in its different forms and how to ameliorate them.

"Hydrogen-air mixtures can ignite with very low energy input, 1/10 that required igniting a gasoline-air mixture. For reference, an invisible spark or a static spark from a person can cause ignition."

"Although the autoignition temperature of hydrogen is higher than those for most hydrocarbons, hydrogen's lower ignition energy makes the ignition of hydrogen–air mixtures more likely. The minimum energy for spark ignition at atmospheric pressure is about 0.02 millijoules."

"The flammability limits based on the volume percent of hydrogen in air at 14.7 psia (1 atm, 101 kPa) are 4.0 and 75.0. The flammability limits based on the volume percent of hydrogen in oxygen at 14.7 psia (1 atm, 101 kPa) are 4.0 and 94.0."

"The limits of detonability of hydrogen in air are 18.3 to 59 percent by volume"[6]

"Flames in and around a collection of pipes or structures can create turbulence that causes a deflagration to evolve into a detonation, even in the absence of gross confinement."

(For comparison: Deflagration limit of gasoline in air: 1.4–7.6%; of acetylene in air,[7] 2.5% to 82%)

Hydrogen is odorless, colorless and tasteless, so most human senses won't help to detect a leak. By comparison, natural gas is also odorless, colorless and tasteless, but industry adds a sulfur-containing odorant called a mercaptan to make it detectable by people. Currently, all known odorants contaminate fuel cells (a popular application for hydrogen). However, given hydrogen's tendency to rise quickly, a hydrogen leak indoors would briefly collect on the ceiling and eventually move towards the corners and away from where any nose might detect it. For that and other reasons, industry often uses hydrogen sensors to help detect hydrogen leaks and has maintained a high safety record using them for decades. Researchers are investigating other methods that might be used for hydrogen detection: tracers, new odorant technology, advanced sensors and others.[8]

Hydrogen leaks can support combustion at very low flow rates, as low as 4 micrograms/s.[9]

"Condensed and solidified atmospheric air, or trace air accumulated in manufacturing, contaminates liquid hydrogen, thereby forming an unstable mixture. This mixture may detonate with effects similar to those produced by trinitrotoluene (TNT) and other highly explosive materials"

Liquid Hydrogen requires complex storage technology such as the special thermally insulated containers and requires special handling common to all cryogenic substances. This is similar to, but more severe than liquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away. (Typically it will evaporate at a rate of 1% per day.[10])

Hydrogen collects under roofs and overhangs, where it forms an explosion hazard; any building that contains a potential source of hydrogen should have good ventilation, strong ignition suppression systems for all electric devices, and preferably be designed to have a roof that can be safely blown away from the rest of the structure in an explosion. It also enters pipes and can follow them to their destinations. Hydrogen pipes should be located above other pipes to prevent this occurrence. Hydrogen sensors allow for rapid detection of hydrogen leaks to ensure that the hydrogen can be vented and the source of the leak tracked down. As in natural gas, an odorant can be added to hydrogen sources to enable leaks to be detected by smell. While hydrogen flames can be hard to see with the naked eye, they show up readily on UV/IR flame detectors. Chemo-chromic indicators can be added to silicone tapes for hydrogen detection purposes. [1]

Hydrogen is extremely flammable.[11] However this is mitigated by the fact that hydrogen rapidly rises and often disperses before ignition, unless the escape is in an enclosed, unventilated area. Demonstrations have shown that a fuel fire in a hydrogen-powered vehicle can burn out completely with little damage to the vehicle, in contrast to the expected result in a gasoline-fueled vehicle.[12]

In January 2007 an explosion of compressed hydrogen during delivery at the Muskingum River Coal Plant (owned and operated by AEP) caused significant damage and killed one person.[13][14]
For more information on incidents involving hydrogen, visit the US DOE's Hydrogen Incident Reporting and Lessons Learned page.[15]

During the 2011 Fukushima nuclear accident, three reactor buildings were damaged by hydrogen explosions. Exposed Zircaloy cladded fuel rods became very hot and reacted with steam, releasing hydrogen.[16][17] The containments were filled with inert nitrogen, which prevented hydrogen from burning in the containment. However, the hydrogen leaked from the containment to the reactor building where it mixed with air and exploded. [18] To prevent further explosions, vent holes were opened in the top of the remaining reactor buildings.

In May 2019 a hydrogen tank exploded killing 2 and injuring 6 in Gangneung, South Korea at the Gangwon Technopark. [26][27]

In June 2019 Air Products and Chemicals, Inc. in Santa Clara CA. The hydrogen manufacturing plant had an explosion during the loading of a tanker truck that was being fueled.[28] This resulted in the shutdown of multiple hydrogen fueling stations in the San Francisco area.[29]

In June 2019 Uno-X fueling station in Norway experienced an explosion,[30] resulting in the shutdown of all Uno-X hydrogen fueling stations and a temporary halt in sales of fuel cell vehicles in Norway.[31] Based on preliminary investigation findings neither the electrolyzer or the dispenser used by customers had anything to do with this incident. Therefor the electrolyzer division will now return to business as usual. [32][33] June 27, 2019 Nel ASA announces the root cause of the incident has been identified as an assembly error of a specific plug in a hydrogen tank in the high-pressure storage unit. [34]